Contamination in industrial water systems is a well known and major problem in a myriad of processes. Boilers, hot water heaters, heat exchangers, steam generators, nuclear power electric systems, combustion engine and diesel coolant systems, evaporator systems, thermal desalination systems, papermaking operations, fermentation processes, ballast water, cruise ship gray water, and the like are a sampling of the types of water systems that are affected by many varieties of contamination. Being a de facto air scrubber, evaporative cooling towers are particularly prone to contamination.
As a consequence of the cooling process, cooling towers flush airborne contaminants into the water phase. These contaminants, for example, can ultimately deposit on the heat exchanger surfaces thus reducing heat transfer. Examples of such contaminants can be particulate matter, organic and inorganic contaminants, oils, process contaminants, and the like. In addition to such contaminants, microorganisms also find their way into the water system. Suspended matter in the cooling water provides the microorganisms with a readily available food source for sustaining life and reproduction. It is well established that the presence of inorganic, organic, and microbiological deposits have a detrimental impact on the operational parameters of industrial or non-industrial water systems in general, resulting in reduced efficiency and increased cost of operation.
Filtration systems are often used to partially remove these particulate contaminants and prevent them from reaching levels that can adversely impact efficient system operation. A majority of the filtration systems currently used for removing suspended solids from industrial water systems are side-stream devices of varying capital cost depending on the complexity of the device. One commonly used method involves drawing an amount of the water in the system (usually a pre-determined % of the recirculation rate in cooling towers, for example) downstream of the recirculation pumps, and then returning the filtered water into the system with the aid of a booster pump. An alternative is to return the filtered water directly to the tower basin, makeup water reservoir, etc. and requires no additional pumping. Generally, only a very small (2-4%) percentage of the recirculation water is pulled into the side stream since a larger volume could result in a significant pressure drop or lowered flow rate that could impact the efficiency of the heat exchanger.
Commonly used sand based or membrane based filtration devices are designed to work at constant filtrate flow. In general, the filtrate flow is maintained constant independent of any fouling that may occur, by increasing the applied pressure. Often, the difference in applied pressure or the difference between the incoming and outgoing pressures can be a trigger for back flushing of the filtration device. The size of the side-stream filtration unit required is calculated using the recirculation rate of the cooling system. Generally, cooling systems are large volume processes and taking the entire volume through a side-stream filter device would require a very large filtration unit which would be impractical and expensive. However, certain small volume systems and once through systems such as irrigation systems can process the total volume through the side-stream filtration device. In an automated filtration system, for example, removal of particulate debris typically results in reduced flow through the unit and the system undergoes a “backwash” flush to remove the collected debris and send it into the waste stream. Peaks of system contamination will increase fouling of the filtration device, and will lead directly to an increase in backwash frequency. Systems with a high level of contamination may require a larger filtration unit or a larger filtration media or a series of filtration devices in order to keep the system clean.
There thus exists an industrial need for improved and more efficient methods of reducing contamination and controlling system parameters in process streams. In addition to keeping the water system cleaner and maintaining the operational efficiency of the process equipment, there are several regulatory and sustainability drivers that create a need for reduction in contaminants within process streams. For example, there is increasing regulatory pressure to reduce water and chemical use, and discharge regulations are becoming more stringent. Side-stream filtration devices have been implicated in reducing the use of certain chemicals, for example oxidizing biocides, which in turn have an impact on the environmental footprint and discharge from the facility.